1. Field of the Invention
The present invention relates to a method of manufacturing a micro-tip used in a scanning tunneling microscope, an atomic force microscope for detecting a very weak force, or the like and a female mold substrate used in the manufacture of the micro-tip, and a method of manufacturing a probe having the micro-tip and the probe and, more particularly, to a method of manufacturing a micro-tip, that allows mass production of a tip, which has a small curvature at its distal end, exhibits excellent characteristics in the above-mentioned application, and can realize a multi-tip structure.
2. Related Background Art
In recent years, a scanning tunneling microscope (to be abbreviated as an "STM" hereinafter) that allows direct observation of the electron structures of atoms on conductor surfaces has been developed (G. Binnig et al., Phys. Rev. Lett., 49, 57 (1983)), and a real space image can be measured at a high resolution independently of single-crystal or amorphous materials. Such STM utilizes a tunneling current that flows across a metal tip and a conductive material when a voltage is applied across them and the tip is brought close to a distance of about 1 nm to the conductive material.
This current is very sensitive to a change in distance between the tip and the conductive material, and changes exponentially. Hence, by scanning the tip to maintain a constant tunneling current, the surface structure of a real space can be observed at a resolution on the atomic order.
Objects that can be analyzed using the STM are limited to conductive materials. However, recently, the STM is often applied to structural analysis of a thin insulating layer formed on the surface of a conductive material.
Furthermore, the above-mentioned device and means allow observation of a medium with low electric power without damaging it since they use a method of detecting a very weak current.
Since the STM can operate in air, extensive studies have been made for its applications to various fields such as observation/evaluation and micropatterning of semiconductors or polymers on the atomic or molecular order (E. E. Ehrichs, Proceedings of 4th International Conference on Scanning Tunneling Microscopy/Spectroscopy, "89, S13-3), an information recording/reproduction apparatus, and the like, using the STM technique.
For example, upon applying the STM to an information recording/reproduction apparatus, the distal end portion of a tip of the STM preferably has a small radius of curvature to attain a high recording density.
At the same time, in view of improvement in function, especially an increase in speed of the recording/reproduction system, it is proposed to simultaneously drive a large number of probes (multi-tip structure). For this purpose, however, tips having uniform characteristics must be manufactured on a single substrate.
An atomic force microscope (to be abbreviated as an "AFM" hereinafter) can measure a three-dimensional pattern image on the specimen surface independently of conductors or insulators since it can detect a repulsive force or attractive force acting on the surface of a substance.
The AFM uses a micro-tip formed on the free end of a thin-film cantilever. As in the STM, in order to increase the resolution of the AFM, the distal end portion of the tip is required to have a small radius of curvature.
As a conventional method of forming the above-mentioned micro-tip, a micro-tip formed by anisotropic etching of monocrystalline silicon using a semiconductor manufacturing process is known (U.S. Pat. No. 5,221,415). In the method of forming a micro-tip, as shown in FIG. 1, a pit 518 is formed by anisotropic etching on a silicon wafer 514 coated with silicon dioxide masks 510 and 512, and is used as a female mold of a tip. After the silicon dioxide masks 510 and 512 are removed, the two surfaces of the wafer 514 are coated with silicon nitride layers 520 and 521 to form a pyramid-shaped pit 522 which is to serve as a cantilever and a micro-tip. After the pit 522 is patterned into a cantilever shape, the silicon nitride layer 521 on the rear surface is removed. Then, a glass plate 530 with a SAW-cut 534 and a Cr layer 532 is joined to the silicon nitride layer 520. Thereafter, the silicon wafer 514 is removed by etching, thus manufacturing a probe which is transferred to a mounting block 540 and is constituted by the silicon nitride tip and the cantilever.
Finally, a metal film 542 serving as a reflection film for an optical lever type AFM is formed.
Also, the following methods are available. That is, in another manufacturing method, as shown in (a) in FIG. 2, a thin film layer on a silicon substrate 611 is patterned into a circular shape, the silicon is etched using the patterned thin film layer as a mask 612, and a tip 613 is formed by utilizing side etching (O. Wolter, et al., "Micromachined silicon sensors for scanning force microscopy", J. Vac. Sci. Technol. B9(2), Mar/Apr, 1991, pp. 1353-1357). In still another manufacturing method, as shown in (b) in FIG. 2, a conductive material 625 is obliquely deposited onto a substrate 621 via a reverse-tapered resist aperture portion 624 of a resist film 622 while rotating the substrate 621, and is lifted off (C. A. Spindt, et al., "Physical properties of thin film field emission cathode with molybdenum cones", J. Appl. Phys., 47, 1976, pp 5248-5263).
However, the conventional micro-tip manufacturing method suffers the following problems.
For example, the conventional micro-tip manufacturing method shown in FIG. 1 suffers the following problems.
(1) Since the silicon substrate used as a female mold of a tip is removed by etching in a later process, it cannot be re-used, resulting in low productivity and high manufacturing cost.
(2) Since the silicon substrate used as a female mold of a tip is etched, deterioration of the tip material and the tip shape on the probe surface due to an etchant, and contamination from the etchant occur.
(3) When a tip for the STM is manufactured by coating the tip surface with a conductive material, since the tip is formed to have a sharp distal end, it is not easy to form a coating of the conductive material. When the tip surface is coated with the conductive material, grain clots of the conductive film appear, and it is difficult to control the grain clots with high reproducibility. In the STM that processes a very weak current, i.e., a tunneling current, it is difficult to obtain stable characteristics.
(4) Furthermore, when a micro-tip is formed on a thin-film cantilever, a reflection film is formed on the entire rear surface of a probe in an AFM, and the cantilever warps due to the film stress of the reflection film.
The conventional micro-tip manufacturing method shown in FIG. 2 suffers the following problem.
(5) Strict process management is required to make constant the etching condition for silicon upon forming a tip, the patterning condition for a resist, the deposition condition of a conductive material, and it is difficult to maintain accurate shapes such as the heights, the radii of curvature of distal ends, and the like of a plurality of micro-tips to be formed.